1. Field of the Invention
The present invention relates to a method and apparatus for completion of a petroleum production well. In particular, the invention relates to a method and apparatus for fracturing and gravel packing multiple production zones in a single downhole trip.
2. Description of the Related Art
Petroleum production from a well bore is often enhanced by a process that is characterized as xe2x80x9cfracturingxe2x80x9d. According to the general principles of fracturing, the fracturing process induces increased fluid flow from the wellbore production face by generating additional cracks and fissures into the zone radiating from the well bore wall. The objective of such additional cracks and fissures is an increase in the production face area. This increased production area facilitates migration of a greater volume of petroleum fluid into the well production flow stream than would otherwise occur from the simple cylinder wall penetration area provided by the original borehole.
Among the known methods of creating or enlarging such cracks and fissures into a fluid production zone is that of forcing liquid into the formation under extremely high pressure. Mixed with the high pressure fracturing liquid are particulates such as coarse sand or fine gravel known as propants. These propants have the function holding open and maintaining the permeability of zone fractures.
Often entrained in the natural flow of petroleum fluid from the geologic formations of origin, e.g. production zones, are considerable quantities of fine sand and other small particulates. If permitted, these particulates will accumulate in the production flow tubing and the region of the borehole where the production flow enters the production tubing. Continued accumulation eventually restricts and terminates production flow.
One well known method of controlling a flow restricting accumulation of such fine particulates is placement of gravel around the exterior of a slotted, perforated, or other similarly formed liner or screen to filter out the unwanted sand. This practice is generally characterized as gravel packing. According to one method of practicing the method, a gravel filter is deposited in the annular space between the production screen and the casing in the form of a fluid slurry. The slurry carrier fluid passes through the screen into the production tubing and returned to the surface. The gravel constituent of the slurry is separated by the screen and deposited in the wellbore, liner or casing around the screen.
Typically, a screen or perforated casing liner is positioned within a borehole casing. The casing is perforated adjacent to the production formation. Packers are set in the annulus between the borehole casing and the casing liner, for example, above and below the production zone. A string of tubing is run inside of the liner assembly in the area of the liner screen. The gravel slurry is pumped from the surface down the internal bore of the tubing string and through a crossover tool out into the packer isolated annulus. From the isolated annulus, the slurry carrier fluid passes through the screen into the liner bore thereby depositing the gravel in the isolated annulus around the screen. From the liner bore, the fluid carrier reenters the crossover tool for conduit past a seal between the tubing exterior and the liner bore. Above the upper packer respective to the isolated annulus, the fluid return flow path is routed into the annulus surrounding the tubing which may be the liner and/or the casing.
After placement of the filtration gravel is completed, the crossover tool is repositioned and the circulation of carrier fluid is reversed to flush residual gravel from the tubing string bore.
In many petroleum producing fields, valuable fluids are found in several strata at respective depths. Often, it is desired to produce the fluids of these several depths into a single production tube. Execution of this desire consequently requires that each of the vertically separate production zones is separately gravel packed.
Gravel packing multiple production zones along the same wellbore traditionally has required that the operating string be lowered into and withdrawn from the wellbore for each production zone. The cycle of entering and withdrawing a tool from a borehole is characterized in the earthboring arts as a xe2x80x9ctripxe2x80x9d. The outer string, containing the packing screens, is assembled from the bottom up in a step by step process. The operator must withdraw the operating string after each zone completion in order to add components to the outer string that are necessary to complete the next higher production zone. This also renders it impossible to pack a zone below a previously packed upper zone. In some instances, this is due to an inability to place the operating string back in the desired location due to restrictions placed in the outer string after packing a zone. In other cases, it is due to an inability to relocate the desired zone and to position the crossover tool ports with sufficient precision.
A prior art gravel packing procedure for multiple production zones may include an outer completion string having a combined slip and production packer for supporting the completion string within the cased well. Disposed below the production packer is an upper closing sleeve and an upper zone screen. An isolation packer is disposed below the upper zone screen and a lower closing sleeve. A lower zone screen is disposed below the isolation packer. A first sealing bore surface is disposed between the production packer and the upper closing sleeve. A second sealing bore surface is disposed between the upper closing sleeve and the upper zone screen. A third sealing bore surface is disposed between the upper zone screen and an isolation packer. A fourth sealing bore surface is disposed at the lower zone screen. A sump or basement packer is disposed below the lower zone screen around a lower seal assembly. In the case of an open hole, inflatable packers would be used in place of the basement packer and isolation packers.
A surface manipulated inner service tool is lowered into a well coaxially within the completion string. The inner service tool may include a plurality of bonded outer seal rings around the outside perimeter of an outer tube wall. Within the outer tube is an inner tube. An annular conduit is thereby formed between the two concentric tubes. The center tube and seal units form an annulus extending from upper ports in the uppermost seal unit to the lower crossover ports extending through the outer conduit formed by the seal units and the center tube. An additional length of seal units extends from the crossover ports downwardly for several feet followed by an extension and an additional set of seal units to a ported sub and lower seal assembly at its lower end.
For the function of opening and closing the closing sleeves, a prior art service tool might include two shifting tools, one above the crossover tool and one below. A single shifting tool may be used but it must be located very close to the gravel pack ports so that the shifting tool can be raised a very short distance, close the closing sleeve, and still have the gravel pack ports within the short distance range.
An upper ball check is provided at the lower terminal end of the center tube to prevent downward flow through the flowbore of the center tube. A lower check valve is provided in the conduit of the seal units to prevent the downward flow of fluids in the annulus and into the flowbore formed by those seal units disposed below the crossover ports. Another ball check valve is provided at the lower terminal end of the seal units.
In operation, the basement packer is lowered into the well and set by a wire line at a predetermined location in the well below the zones to be produced. The completion string is then assembled at the surface starting from the bottom up until the completion string is completely assembled and suspended in the well up to the packer at the surface. The production screens are located in the completion string relative to the casing perforations and the basement packer. The inner service tool is then assembled and lowered into the outer completion string. The service tool includes one or more shifting tools, depending upon the number of production zones to be produced, for opening and closing the closing sleeves, When the service tool is lowered into the completion string, the shifting tool opens all of the closing sleeves in the completion string. Therefore, it does not matter whether the closing sleeves were initially in the open or closed position since the shifting tools will move them all to the open position as they pass downwardly through the completion string. Subsequently, these sleeves may be moved to the closed position to set the isolation packer depending on the operational type of packer. The packer assembly and setting tool are then attached to the upper ends of the service tool and completion string and the entire assembly lowered into the well on a work string onto the basement packer.
In gravel packing the lower production zone, the setting tool is disconnected from the completion string and is raised such that the set of upper seals no longer engage the first bore seal of the production packer. At that time, the seals on the upper seal units engage the first, third and fourth bore seals and the crossover ports are adjacent the lower closing sleeve which is open. In order to set the isolation packer, the lower closing sleeve must be closed. To do so, the shifting tool in the service string is utilized so that the annulus between the closing sleeve and the outside of the service tool may be pressurized to set the isolation packer.
Next, gravel slurry is pumped down the flowbore of the work string and center tube. The ball valve directs the gravel through the crossover ports and through the open closing sleeve into the lower annulus. The gravel accumulates in the lower annulus adjacent the sump packer with the return flowing through the lower zone screen and ported sub. The return flow continues up the flowbore of the lower seal units and through the lower ball valve. The return flow then passes through the bypass apertures around the crossover ports and up the annulus. Thereafter, the return flows out through the upper ported sub and up the upper annulus formed by the work string and outer casing.
Upon completing the gravel pack of the lower production zone, fluids are reverse circulated d own to the crossover ports to flush residual fluids remaining in the flow bores. Fluid is then pumped down the annulus between the work string and casing, through the upper ported sub at the upper end of the seal units, down the annulus and through the bypass apertures around the crossover ports. The lower ball check prevents the fluid from passing down into the flowbore of the lower seal units and directs the flow through upper ball check and flowbore to the surface.
In gravel packing an upper production zone, the service tool is raised such that the crossover ports are adjacent the upper closing sleeve. Also, the seals on the seal units engage the first, second, and fourth seal bores. Circulation and reverse circulation occurs substantially as previously described with respect to the lower production zone.
A disadvantage of the prior art as described above is that the prior art method and apparatus does not permit performing the gravel pack in a weight-down position which is preferred in the industry. The work string is made up of steel tubing which will contract and expand in the well, particularly when the work string is several thousand feet long. At such lengths, the steel stretches causing the lowermost end of the work string to move several feet within the well. This is particularly a problem in gravel packing operations when it is necessary to position the gravel pack ports accurately across from the closing sleeves.
It is also advantageous to perform other operation, such as hydraulic fracturing, in a down weight position. The work string extending from the top of the service tool to surface has substantial movement during a fracturing or gravel packing operation. The movement of the work string is even more exaggerated than during a gravel pack operation due to the thermal effects caused by the cool fracturing fluid being pumped down through the work string at a very high rate. This tends to cause shrinkage in the work string Further, the work string tends to balloon due to the increased pressure within the work string which also causes the work string to shrink. These combined affects tend to shorten the work string substantially during the operation.
Although a weight indicator is used at the surface to determine the amount of weight hanging off the crown block, the fact that the weight appears to be staying the same does not provide an indication as to whether the length of the work string is changing at its lower end. If the work string shrinks several feet, the gravel pack ports may be raised a distance so as to cause the gravel pack ports to the moved up into the packer seal bore and prematurely end the operation.
Another problem during the fracturing or gravel packing operation is that the pumping of the fluid through the work string at a very high rate causes a vibration in the work string thereby causing it to move up and down. With a very long work string, this reciprocal motion may get very large causing it to bounce up and down within the well such that it may act like a spring.
The present invention provides an apparatus and method of manipulating the apparatus for sequentially fracturing and gravel packing several production zones at respective depths along a cased borehole. Characteristically, the invention provides for the complete and selective isolation of each production zone. Moreover, the invention permits the well completion operation to be accomplished in a single xe2x80x9ctripxe2x80x9d cycle into the well.
One object of the present invention is to have the capability of gravel packing multiple zones in a multiple zone completion string with a single trip into the well of the service tool and also have the ability to set weight-down on the completion string during the treatment of the production zones
Initially, the raw borehole of a well is lined with a steel casing pipe. Next, the casing pipe is perforated at one or more locations adjacent to respective production zones. A basement packer is thereafter set by wireline below the lowermost production zone. A completion string is assembled with production screens positioned along the completion string length, relative to the basement packer location, to align with each production zone. Each screen may be selectively opened and closed by means of an axially sliding sleeve. Annulus packers are placed in the completion string above and below the perforated casing sector respective to each production zone. Also in the completion string respective to each production zone is a fluid transfer orifice that may be selectively opened and closed by means of an axially sliding sleeve. Finally, each production zone segment of the completion string includes at least one appropriately positioned indicating coupling for manipulating a xe2x80x9cSMARTxe2x80x9d collet in a cooperative service string.
As the assembled completion string hangs from the rig table down into the casing mouth, the service string is assembled coaxially into the completion string. At its lower end, the service string includes, in series, a lower shifting tool, the SMART collet and an upper shifting tool. Above the collet and shifting tools is a cross-flow section. A stand of wash pipe spaces the cross-flow section below the setting tool. The setting tool joins the service string to the work string (drill string) in a manner not subject to downhole disassembly. However, the setting tool also joins the service string to the completion string but in a manner that allows the service string to be disconnected from the completion string by surface manipulation such as rotation.
The completion assembly is lowered into the well and seated onto the basement packer joint. The drill string is then rotated to release the service string from the completion string to permit axial repositioning of the service string relative to the completion string.
Starting from the lowermost production zone and progressing upwardly, the service string is raised to align the cross-over flow port with the first isolation packer. When aligned, the drill string flow bore is pressurized with working fluid to set the first isolation packer against the casing. Next, the closure sleeves respective to the fluid transfer orifice and production screen are opened and the service string aligned to transfer fracturing fluid into the zone isolated annulus between the casing and the outside surface of the completion string. The fracturing fluid initially begins with a substantially xe2x80x9cpurexe2x80x9d fluid and concludes with gravel particles entrained in the fluid.
The isolation packers respective to each production zone are set independently of other packers or tools. When the gravel packing procedure for each production zone is completed, the service string is lifted and realigned in a weight-down procedure by means of the smart collet. Such resetting of the service string directs a reverse circulation of xe2x80x9cpurexe2x80x9d fluid from the casing annulus into the service string flow bore-to flush the flow bore of residual gravel slurry.
Following the reverse flow flushing, the closing sleeves respective to the fluid transfer orifices and production screen are closed and the service string lifted to accommodate the next higher production zone where the procedure is repeated.
Sequentially, each production zone is fractured, gravel packed and returned to pressure isolation. Consequently, each zone may be treated at a pressure that is appropriate for that particular production zone. Moreover, each zone may thereafter be selectively produced.